As is well known, a femtocell is a small-sized cell having a radius which ranges from 10 to 20 m. The concept of a femtocell is distinguishable from the concept of a macrocell which is a typical cell supported by a service provider. In general, the femtocell is designed to support being used in a small-sized apace such as a home or small business.
A micro-base station is installed in an indoor area such as a home or an office. The micro-base station is integrated with a network, so that it ensures mobility and the transmission of a large amount of data, expands the service areas available to mobile communication, improves the performance of a user service, increases the capacity of a base station, and supports a variety of inexpensive mobile communication services for a user. Furthermore, in an environment where the radius of a cell has been limited to within a minimum of space such as a room of a home or an office, the micro-base station actively responds to a user's demand, reduces the Capital Expenditure (CAPEX)/Operational Expense (OPEX) of a service provider by reducing the time it takes to install an additional cell and the operating cost, creates a new market for equipment manufacturers, and provides a wireless environment in which the improvement of the quality of the user service has been taken into consideration.
Such micro-base stations are installed in response to users' demands rather than the planned design of service providers. Accordingly, when a micro-base station is additionally installed, there is required a technology for base station self-establishment in which the initial base station self-configuration of creating installation parameters is performed on a basis of the internal configuration and a technology for base station self-configuration in which the identification of an adjacent base station, the setting/registration of relations and the setup of a connection to a core network are performed prior to operation of the base station. Additionally, there needs a technology for base station operation self-optimization which controls the intensity of signals from the base station and which optimizes handover parameters using information about the types of signals and traffic between the base station and a surrounding base station.
Furthermore, femtocells support the concept of a Closed Subscriber Group (CSG) which allows only the users of a group of authorized users to enter a network. Femtocells that support the concept of a CSG may assign unique IDs, which are referred to as CSG IDs. A femtocell cannot transmit a wireless signal until installation and configuration have been finished. If serious spectral interference occurs in the surroundings after the femtocell was installed, the femtocell may stop providing service. Furthermore, the reconfiguration of the network of a service provider resulting from the installation of a new femtocell is prohibited, and the influence that the installation causes must be minimized.
A micro-base station supports three user access modes, which are open mode, closed mode, and hybrid mode. First, open access mode allows service to all subscribers regardless of the state of subscription to a CSG, and is the same as the operating mode of a macrocell base station when the concept of a femto-base station is not applied. Second, closed user access mode provides service only to subscribers to a CSG, and a cell which supports such closed user access mode is referred to as a CSG cell. Finally, hybrid user access mode allows a CSG subscriber closed operation, and allows a CSG non-subscriber open operation, and is a mixture of the above two modes. When users in two modes are connected, service is provided to a CSG subscriber first.
A CSG cell includes a CSG ID in system information, and then broadcasts the system information. Optionally, a CSG cell may broadcast a CSG designator. A CSG cell can be identified by obtaining a Physical Cell ID (PCI), that is, a cell ID in a physical layer, during a process where the CSG cell is being synchronized with a preamble in the physical layer. Since the PCI is identical to the value of the preamble defined in the physical layer, the PCI can be obtained while the terminal is being synchronized with the cell. The range of the PCIs of CSG cells has been reserved by a system or a service provider.
The service provider may adjust especially some cell selection parameters, such as the Q-offset or Q-hyst, for a specific CSG cell. This may be used as a strategy to fix the selection of a cell to a CSG cell in order to overcome a ping-pong problem in which a terminal repeats the selection of a cell and the changing of a cell between a macrocell and a CSG cell. Furthermore, that may also be used as a strategy for inducing a terminal near a CSG cell to select the CSG cell as a cell in order to solve the problem of interference between cells.
Hybrid user mode may be adapted to provide the CSG service to employees within a building, such as a shopping mall, and to provide open/shared service to customers. In this case, a shopping mall owner may make a special contract for fees and use with a communication service provider. Not only can the owner of a micro-base station provide inexpensive communication service to employees, but the communication service provider can also achieve the effect of eliminating a shading area or the expansion of coverage. Accordingly, as described above, the hybrid user mode enables mutual benefits to be achieved.
Recently, a hybrid cell is attracting attention as a method for avoiding interference. A terminal other than a CSG subscriber terminal is located near a CSG cell, access to the CSG cell is not allowed, and therefore the terminal continuously stays in a macrocell. In this case, the upstream link signal of the terminal causes serious interference to the CSG cell. Accordingly, if such a terminal is allowed to enter a CSG cell, this can result in the effect of considerably avoiding interference. This indicates that the concept of a hybrid cell is not only used in a company such as a shopping mall, but is also used in a home in order to reduce the interference which is imposed on a user.
A base station which operates in hybrid access mode should minimize the influence it has on existing CSG subscribers. That is, when a hybrid cell has allowed a non-subscriber other than a CSG subscriber to enter itself for the reason of coverage or interference, it has to ensure the normal processing of an access request when an existing CSG subscriber makes an access request. In this case, the data transfer rate of an existing terminal operating in hybrid access mode should be reduced, the existing terminal should be handed over to another site, or, in some cases, the access of the existing terminal may be cancelled.
When a terminal receives a rejection notification from a CSG cell when attempting to access the cell, the terminal should delete the CSG ID of the corresponding cell from an allowable CSG list which is managed by the terminal. In contrast, when a terminal obtains permission from the CSG cell to access a CSG cell which is absent from an allowable CSG list (which is possible in the case of the manual selection of a CSG cell), the terminal should newly add the CSG ID of the corresponding cell to the CSG list.
The use of a femtocell must be the same as the use of a typical macro-base station in terms of user experience. Furthermore, the load of registration or paging attributable to the use of a femtocell should be minimized. Furthermore, the use of a femtocell must not influence the performance, coverage and capacity of an existing base station.
As described above, a femtocell or a micro-base station defines three operating modes: open mode which allows all terminals to have access, closed mode which allows only authorized subscribers to have access, and hybrid mode which provides discriminative service to a non-subscriber. A femtocell which operates in closed mode does not need to provide service to an unconcerned terminal.
However, when cells which operate in closed mode are densely deployed, a non-subscriber terminal should communicate with a relatively remote macrocell and therefore strong power is required. Interference attributable to the strong power incurs a reduction in QoS which is provided to a micro-base station subscriber. Furthermore, there is a problem in that a non-subscriber terminal undergoes an interruption in a call or cannot appropriately receive a control channel because of the interference of a surrounding base station in closed mode.